The evolution of QSO clustering is investigated with a new sample of 388 QSOs with 0.3 < z \leq 2.2 , B \leq 20.5 and M _ { B } < -23 .
Evidence is found for an increase of the clustering amplitude with increasing redshift .
These measurements allow to further distinguish among the various physical scenarios proposed to interpret the QSO phenomenon .
A single population model is inconsistent with the observations .
The general properties of the QSO population would arise naturally if quasars are short-lived events connected to a characteristic halo mass \sim 5 \cdot 10 ^ { 12 } M _ { \odot } .